Question: Which of these numbers is composite? ${3,\ 9,\ 11,\ 17,\ 53}$
Explanation: A composite number is a number that has more than two factors (including 1 and itself). 3, 11, 17, and 53 each have only two factors. The factors of 9 are 1, 3, and 9. Thus, 9 is the composite number.